How It Sees

Cosmic graffiti (1)
Each of the WFPC2's four cameras takes two images with each of its four
filters for a good reason. Most of the "snowflakes" you see here are cosmic
rays—atomic nuclei whizzing about space at close to the speed of
light—that struck the CCD at the moment the image was made. Each cosmic
ray leaves a streak across the CCD. Like scratches on a photographic negative,
these streaks interfere with the scene we want to see, namely, the Eagle
Nebula. Fortunately, cosmic rays arrive randomly, so the marks they leave
differ from image to image. Click between the two "Cosmic Graffiti" images, which
were taken within moments of each other. Notice how the cosmic-ray marks differ
in each.